reuse existing capacity in sbo_buffer::grow

[Ramya-9353]

Repro: a 200-element [1.25,...] array (1001 bytes) fed to stream_parser a byte at a time with number_precision::precise, so every number straddles a write_some boundary, makes 813 global operator new calls totalling 56.1 GB with a largest single block of 1.14 GB; a 32 KB body in 4 KiB chunks peaks at a 2.2 MB block, and that peak keeps doubling as the body grows.
Cause: grow never checks whether the buffer already has room, so every append reallocates and each one floors the new capacity at old_capacity * 2, while clear keeps the block, so the doubling carries across numbers and across reset, and the inline buffer goes unused past the first append.
Fix: return early when the requested size fits the current capacity, matching what string_impl::append already does; the two runs above then total 19.8 kB and 240 kB, with largest blocks of 6 kB and 48 kB.

[cppalliance-bot]

An automated preview of the documentation is available at https://1178.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-07-30 07:23:56 UTC

[cppalliance-bot]

GCOVR code coverage report https://1178.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1178.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1178.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-07-30 07:38:54 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (6a57776) to head (a9b4402).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1178   +/-   ##
========================================
  Coverage    93.92%   93.92%           
========================================
  Files           91       91           
  Lines         9279     9285    +6     
========================================
+ Hits          8715     8721    +6     
  Misses         564      564           

Files with missing lines	Coverage Δ
include/boost/json/detail/sbo_buffer.hpp	100.00% <100.00%> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1178/pullrequest.html

[grisumbras]

Thank you for catching this.
Slightly changed and merged via 5779b03.